askrene-bias-channel: bias call add up.

The channel bias feature is not being used yet by any plugin, so this
hopefully doesn't break any working code.
When askrene-bias-channel is called the bias quantity is added on top of
any previous biased already present on that channel instead of
overwriting it.

Changelog-Changed: askrene-bias-channel: bias call add up.

Signed-off-by: Lagrang3 <[email protected]>

contrib/msggen/msggen/schema.json

@@ -281,7 +281,7 @@
       "title": "Command to apply a manual bias to a channel in a layer",
       "added": "v24.11",
       "description": [
-        "The **askrene-bias-channel** RPC command tells askrene to favor or disfavor a channel when considering it for routing."
+        "The **askrene-bias-channel** RPC command tells askrene to favor or disfavor a channel when considering it for routing.  Repeated calls to askrene-bias-channel add up the biases up to +100 or down to -100."
       ],
       "request": {
         "required": [

doc/schemas/lightning-askrene-bias-channel.json

@@ -5,7 +5,7 @@
   "title": "Command to apply a manual bias to a channel in a layer",
   "added": "v24.11",
   "description": [
-    "The **askrene-bias-channel** RPC command tells askrene to favor or disfavor a channel when considering it for routing."
+    "The **askrene-bias-channel** RPC command tells askrene to favor or disfavor a channel when considering it for routing.  Repeated calls to askrene-bias-channel add up the biases up to +100 or down to -100."
   ],
   "request": {
     "required": [

plugins/askrene/layer.c

@@ -10,6 +10,9 @@
 #include <plugins/askrene/layer.h>
 #include <wire/wire.h>
 
+#define MIN(a, b) ((a) < (b) ? (a) : (b))
+#define MAX(a, b) ((a) > (b) ? (a) : (b))
+
 /* Different elements in the datastore */
 enum dstore_layer_type {
 	/* We don't use type 0, which fromwire_u16 returns on trunction */
@@ -294,15 +297,18 @@ static const struct bias *set_bias(struct layer *layer,
 		bias = tal(layer, struct bias);
 		bias->scidd = *scidd;
 		bias_hash_add(layer->biases, bias);
+		bias->bias = 0;
 	} else {
 		tal_free(bias->description);
 	}
-
-	bias->bias = bias_factor;
+	int bias_sum = bias->bias + bias_factor;
+	bias_sum = MIN(100, bias_sum);
+	bias_sum = MAX(-100, bias_sum);
+	bias->bias = bias_sum;
 	bias->description = tal_strdup_or_null(bias, description);
 
 	/* Don't bother keeping around zero biases */
-	if (bias_factor == 0) {
+	if (bias->bias == 0) {
 		bias_hash_del(layer->biases, bias);
 		bias = tal_free(bias);
 	}

tests/test_askrene.py

@@ -1255,7 +1255,8 @@ def amount_through_chan(chan, routes):
                             total += p['amount_msat']
                 return total
             amount_before = amount_through_chan(chan, route['routes'])
-
+            # remove any previous biases
+            l1.rpc.askrene_age('biases', int(time.time()) + 10)
             l1.rpc.askrene_bias_channel('biases', chan, -bias)
             route2 = l1.rpc.getroutes(source=l1.info['id'],
                                       destination=nodeids[n],